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Today's Topics:
1. Re: Trouble Understanding How to Start (David McBride)
2. Functors and Applicatives; I'm just not getting it ... (Frank)
3. Re: Functors and Applicatives; I'm just not getting it ...
(Adam Flott)
4. Re: latest Haskell Platform build fails (Heinrich Apfelmus)
5. Re: Functors and Applicatives; I'm just not getting it ...
(Karl Voelker)
6. Re: Functors and Applicatives; I'm just not getting it ...
(Benjamin Edwards)
----------------------------------------------------------------------
Message: 1
Date: Sat, 18 Oct 2014 15:32:47 -0400
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Trouble Understanding How to Start
Message-ID:
<can+tr43cmgksa4hxpguhau5dxzqqnavkqf+e1k3o-8fb7e+...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
It might help you to remember that you can sort of replace the types in a
function with their equivalents, and it may lead you to realizing how to do
them.
For example:
get :: Name -> Env -> Int
is equivalent to
get :: String -> (String -> Maybe Int) -> Int
So think about how you can take the string you have, put it into that
function that takes a string and gives a maybe int, then how to get the int
out of the maybe int. You'll get it.
On Sat, Oct 18, 2014 at 11:08 AM, Keeley Abbott <[email protected]>
wrote:
> Hi All,
>
> So, I am taking a Functional Programming class this fall, and I have
> struggling to understand how to build types and functions correctly. For
> some reason it just isn?t clicking. I have the following module I am
> working on, and I need some guidance.
>
> ? Variable names
> type Name = String
>
> ? An environment for looking up the value of a variable
> type Env = Name -> Maybe Int
>
> ? An empty environment
> empty :: Env
> empty env = Nothing
>
> ? Set a variable to a value in the environment if it doesn?t already exist
> set :: Name -> Int -> Env -> Env
> set x i e = \y -> if x == y then (Just i) else e y
>
> ? Lookup the value of a variable in the environment
> get :: Name -> Env -> Int
> get x env = ?? (I previously had ?type Env = Name -> Int?, so this was
> working with env x, but we had to change empty to be something other than
> just a runtime error, which is why I made Env = Name -> Maybe Int)
>
> ?Remove a variable from the environment
> unset :: Name -> Env -> Env
> unset x e = \y -> if x == y then empty x else env y
>
> At this point I don?t know how to fix get, so I get an int to display (it
> keeps telling me it can?t match the type ?Maybe Int? with the expected type
> ?Int?). And I don?t know where to go from there to create getOr, setAll,
> and mapEnv functions, because I am just not understanding what I am doing.
> For the most part I have been doing simpler things that I could just mess
> with the functions until they work, but even at that I?m not getting HOW
> they work? Any assistance or instruction on how I can GET what I am trying
> to do would be greatly appreciated.
>
> Thanks,
> Kallarhynn
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 2
Date: Sat, 18 Oct 2014 18:37:18 -0400
From: Frank <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Functors and Applicatives; I'm just not
getting it ...
Message-ID:
<ca+a3wklxtifmqhhvsxens8noq475hp+lslo-hoooxvsnfoc...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I've had a go at LYAH and CIS 194 and the Typeclassopedia and I just don't
get get functors and applicatives. I'm simply not understanding them, what
the various symbols/keywords mean, what they represent, how to think of
them, etc. Nothing. Is there any kind of documented model I should be
considering? Is there a "functors and applicatives for Dummies" I should
read? Should I just give it up, not bother with Haskell and just stick to
scheme/ruby/C++?
--
P.S.: I prefer to be reached on BitMessage at
BM-2D8txNiU7b84d2tgqvJQdgBog6A69oDAx6
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Message: 3
Date: Sat, 18 Oct 2014 18:45:53 -0400
From: Adam Flott <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Functors and Applicatives; I'm just
not getting it ...
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
On Sat, 18 Oct 2014 18:37:18 -0400
Frank <[email protected]> wrote:
> Is there a "functors and applicatives for Dummies" I should
> read? Should I just give it up, not bother with Haskell and just stick to
> scheme/ruby/C++?
>
Give this visual tutorial a whirl,
http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html
And then play around with
http://hackage.haskell.org/package/optparse-applicative
Those 2 resources helped reinforce my understanding.
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Message: 4
Date: Sun, 19 Oct 2014 09:55:38 +0200
From: Heinrich Apfelmus <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] latest Haskell Platform build fails
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
Mateusz Kowalczyk wrote:
> On 10/18/2014 01:16 PM, Michael Martin wrote:
>>
>> I find it astonishing that build/install fails because it finds
>> that a dependency that it needs is already installed. This is
>> totally unreasonable. I was really looking forward to experimenting
>> with Cloud Haskell, but the pain of installation, coupled with
>> Haskell's well-known issues with "dependency hell", have soured me
>> on Haskell. The language itself is amazing - I really like it. But
>> I'm afraid that the dysfunctional nature of the Haskell ecosystem
>> is driving me back to Erlang/OTP. OTP has proven to be industrial
>> strength. I hope that someday (soon), Haskell will be able to claim
>> that, as well.
>>
>
> Why don't you use [1] instead of getting the source binary and
> finding yourself struggling? It comes with GHC.
>
> [1]: http://www.haskell.org/platform/linux.html#binary
Indeed. Michael, don't make this harder on yourself than it needs to be.
I know, you're using Linux, but it's just easier to download the binary
and run the installer. Trying to compile source code in an ecosystem
that you are not familiar with yet is likely to give you headaches. The
download site for GHC explicitly says "Stop! For most users, we
recommend installing the Haskell Platform instead of GHC" in big red
letters.
Also, Michael, you don't actually have to install the platform. You
*already have* a working Haskell environment, as you managed to install
GHC and cabal just fine. Besides the packaging, the Haskell platform
just contains some useful packages, which you can easily install after
the fact via cabal.
Best regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
------------------------------
Message: 5
Date: Sun, 19 Oct 2014 01:20:07 -0700
From: Karl Voelker <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Functors and Applicatives; I'm just
not getting it ...
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
I suggest that you ignore applicatives for now and just focus on plain-old
functors. They are the simplest part, and once you are confident in dealing
with them, adding on applicatives will be much easier.
And, although it can be difficult when you are really lost, if you can ask some
more specific questions, this list will provide plenty of answers.
-Karl
> On Oct 18, 2014, at 3:37 PM, Frank <[email protected]> wrote:
>
> I've had a go at LYAH and CIS 194 and the Typeclassopedia and I just don't
> get get functors and applicatives. I'm simply not understanding them, what
> the various symbols/keywords mean, what they represent, how to think of them,
> etc. Nothing. Is there any kind of documented model I should be considering?
> Is there a "functors and applicatives for Dummies" I should read? Should I
> just give it up, not bother with Haskell and just stick to scheme/ruby/C++?
>
> --
> P.S.: I prefer to be reached on BitMessage at
> BM-2D8txNiU7b84d2tgqvJQdgBog6A69oDAx6
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 6
Date: Sun, 19 Oct 2014 01:46:46 -0700 (PDT)
From: "Benjamin Edwards" <[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Cc: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: Re: [Haskell-beginners] Functors and Applicatives; I'm just
not getting it ...
Message-ID: <1413708406546.e64bc258@Nodemailer>
Content-Type: text/plain; charset="utf-8"
The only thing I feel to recommend given the generality of the question is to
say: Pick some concrete instances of Functor, and play with them. Write a
program with a failing component using 'Maybe' then see how much you can get
done without pattern matching. Write a program over some lists and use fmap
instead of map. Realise there isn't much more to fmap than a very general
signature and some laws. See what the laws mean for the two concrete instances
that you just used. Be satisfied. When you get a little further on your journey
and see more interesting examples of the class you will see how useful it is.
For instance most parser interfaces admit a functor instance. Which means that
say you had a language with variable identifiers with a leading '$' sign
followed by a string, writing a parser is super simple. Assume 'stringVar' is
a 'Parser String' that parsers an identifier and 'Var' is a new type on strings
for wrapping identifiers then Var `fmap` stringVar is a 'Parser Var'. I
could offer again that a functor is nothing more than a principled way to
change the 'generic type' of things that look like containers of type Foo<T>,
not sure if that is helpful, given your previous reading list.
I hope this offers some insight. Haskell, like any learning endeavour requires
you to just mess about with it until things start to click. You will miss out
on a very rich learning experience if you cast it aside. Good luck!
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